The present invention relates to a pneumatic die cushion apparatus.
A pneumatic die cushion apparatus of conventional type is shown in FIG. 7. In the figure, reference numeral 1 designates an air cylinder, which is separated by a piston 2 into an upper chamber 1U and a lower chamber 1L. The upper chamber 1U is opened to the atmospheric air through an opening 5. Reference numeral 4 designates a wear plate fixed on the upper end of a piston rod 3 and receives the cushion pin (not shown). Reference numeral 6 designates a buffer tank, and this communicates with the lower chamber 1L through a connection pipe 7.
In a die cushion apparatus of such structure, the pressure Pu in the upper chamber is equal to the atmospheric pressure and is constant because the upper chamber 1U is opened to the atmospheric air. Consequently, the die cushion capability or force F is obtained from the pressure Pl in the lower chamber and the sectional area Al of the piston 2: F=P.multidot.A1. However, because the pressure Pl in the lower chamber is increased when the piston 2 moves down, i.e. when the volume of the lower chamber 1L is decreased, the die cushion capability increases. On the other hand, die cushion capability or force, i.e. the proper blank holding pressure (F) necessary for the press fabrication, is determined by the quality of the material, and it is inconvenient when the force increases too much.
For this reason, the conventional type apparatus is provided with a buffer tank, which has a volume 5-8 times as large as the volume of the cylinder 1. This avoids an extreme increase of the die cushion force F by enlarging the apparent or effective volume of the lower chamber 1L.
Therefore, as shown for the sake of comparison with the present invention in FIG. 2 by the two-dot chain line, the necessary blank holding pressure Fp can be obtained as soon as the piston 2 begins to go down from the upper limit UL if the initial pressure P1S necessary for obtaining the die cushion capability (the proper blank holding pressure) Fp is established in the lower chamber 1L and the buffer tank 6. Thereafter, the pressure gradually increases as shown by the one-dot chain line in FIG. 2. Thus, the die cushion capability when the piston 2 reaches the lower limit LL is Fpe.
As described above, the die cushion apparatus of the conventional type is constructed in such manner as to increase the effective volume of the lower chamber 1L by introducing a large-capacity buffer tank and to obtain the proper blank holding pressure at a constant level.
With the introduction of large-size presses and the automated equipment such as transfer presses, and with the increasing demand for higher product quality and productivity, the following problems arise:
(1) The space required for the buffer tank 6 is large and not very economical, and this also hinders the installation of a large-size press and the layout of the other functional components. Particularly, it causes significant problems in a transfer press equipped with a large number of dies. PA1 (2) Much time is required for establishing the initial pressure P1S or for an adjustment to increase the die cushion capability. This has the drawback that, even if the other initial conditions can be met within short time, the press cannot be operated quickly. On the other hand, to establish the initial pressure P1S quickly, a high-pressure large-capacity compressor must be installed, and this is disadvantageous due to the expense and the space requirements. PA1 (3) Further, even when the disadvantages of (1) and (2) above are accepted, it is impossible to increase the capacity of the buffer tank 6 infinitely. Therefore, the die cushion capability inevitably increases more or less as the piston 2 goes down. PA1 (4) In addition the materials for press fabrication have become increasingly complicated and sophisticated from the viewpoints of cost reduction or the quality improvement of the end products. In some cases, defective products may be produced depending upon the forms of the materials or fabrication unless the die cushion for press fabrication is changed adequately according to the circumstances. In other words, if the die cushion capability can be properly changed and adjusted during the press fabrication, a wide variety of products can be fabricated at high efficiency, and extensive cost reduction along with the production of high quality products can be actualized. However, this cannot be accomplished by conventional pneumatic die cushion equipment.
Also, for providing an adjustment to decrease the die cushion capability, a quick-acting large-size exhaust valve must be installed and this also causes an economic burden. Since a large quantity of high pressure air must be released to the atmosphere air or supplied for each adjustment, this leads to an additional economic disadvantage.
However, the higher quality and the cost reduction are in demand these days, and the increase of the die cushion capability itself is no longer acceptable in many fields of the fabrication industry.